1. Field of the Invention
This invention relates to improvements in air brake control systems for controlling the braking functions of multi-axle heavy-duty vehicles, including tandemly towed vehicles, and, more particularly, relates to an improved modular system for use in connection in each axle of a trailer or trailers while being towed behind tractor vehicles.
2. History of the Related Art
This application is an improvement of the air brake valve system disclosed in U.S. Pat. No. 4,593,954 issued Jun. 10, 1986 to Sergio Campinini, the entire contents of which are incorporated herein by reference. The present control valve system is designed to provide trailer spring brake control as required by DOT (Department of Transportation) FMVSS No. 121 as modified by Docket No. 90-3, Notice 2. This notice eliminated the requirement for a "protected tank" for release of the spring brakes in an emergency situation. The notice also requires that no single leakage type failure in the service system such as a rupture of the service reservoir shall result in a supply line pressure falling below a threshold pressure of 70 psi. The regulations further require that the spring brakes shall provide no retardation force when the supply line pressure is 70 psi or greater.
The control valves of the present invention also retain maximum system pressures in the spring brake chambers until the supply line pressures fall to a predetermined isolation pressure below the threshold pressure, at which time the spring brakes exhaust to atmosphere and are thus applied. The isolation from line pressure prevents brake drag which could result from varying supply line pressures. The valves of the present invention also provide an integrated quick release function which will meet the three second parking brake requirements of DOT FMVSS 121 with either two or four spring brakes attached.
The control valves of the present invention also provide an anti-compounding function to prevent the dual application of the service brakes in addition to the parking brakes. A problem can occur in prior art anti-compounding systems wherein any service air pressure is conveyed through a twoway check valve arrangement to apply pressure to reduce the spring brake force to the same extent that service brake force is applied to the brake pedal by the vehicle driver. With such prior art systems, when the vehicle considered is the second trailer in a "doubles" or "tandem" rig, and the driver, when connecting the air lines, turns on the service line but connects and fails to turn on the supply line, a dangerous situation is created in the event the driver subsequently releases his parking brakes but is required to remain still with his service brakes applied for an extended time. In this condition, the current method of anti-compounding by sending service air to the spring brakes would charge the spring brakes and possibly the reservoir to whatever pressure was applied by the driver's foot pedal. This pressure might be adequate to release the parking brakes to a degree that would allow the vehicle to be driven away. If this should happen, without the supply line being turned on, the brakes could easily apply partial pressure to the point of starting a fire, wearing away the brake linings, or reducing available service braking below a safe level due to the lack of pressure in the service reservoir which would not have been properly supplied.
New DOT regulations require that the valves in such braking systems be designed to protect and maintain the supply line pressure at a level that will allow release of the spring brakes for quick removal of a disabled vehicle. During emergency situations, such as when an accident involves the disruption of the air supply to a trailer, the air pressure which has been employed to release the emergency brake system is lost and the brakes are automatically applied by spring pressure. The most common causes for disruption of the trailer air supply are damage to air connectors or the gladhands at the front of the trailer, damage to the air line from the connector to the supply reservoir, damage to the reservoir itself, failure of a brake chamber or of the supply in a hose. In each of these conditions, the brakes are automatically set by spring pressure preventing the vehicle from being moved without mechanically caging the springs by inserting a special tool into the brake chamber and retracting the spring by screwing a nut onto the tool. Such caging must be done on each brake chamber.